In a wire-cut electroerosion apparatus for performing electrical discharge machining, during machining, a wire electrode is continuously fed from a wire bobbin to a machining slit at a predetermined rate of travel. Therefore, during automatic machine operation over a long period of time, the bobbin may run short or empty, and it will become necessary to change over to a spare wire bobbin and to feed or refeed new wire from the spare bobbin into the machining slit.
To change over from a first wire, unwound from a first bobbin over to new wire from a spare wire bobbin without joining the first wire to the new wire, a wire-cut electroerosion apparatus, such as described in Japanese Laid-Open Patent Publication No. 59-8499, may be provided having, for example, two wire transport paths, each of which includes a respective brake device, a pair of feed rollers, a rod electrode for melting a wire, and an upper wire guide. The first wire is supplied along one of the wire travel paths to the inlet of a suction pipe and the suction pipe guides the wire through a workpiece to a pair of receiving rollers. After cutting the desired contour on the workpiece using the first wire, the wire is cut and new wire from a spare bobbin may be supplied along the other wire transport path and guided through the workpiece to the pair of receiving rollers by means of the suction pipe.
Japanese Laid-Open Patent Publication No. 63-139616 also discloses a wire-cut electroerosion apparatus having multiple wire bobbin mounting means, a wire tensioning means, and a wire supply means including wire guide means for automatically positioning and supplying multiple wire electrodes. Further, the apparatus is provided with wire guide moving means for attachably and movably supporting one of multiple wire supplying means whereby a wire electrode to be used may be automatically and selectively changed.
In each of the above disclosures, two or more wire transport paths are provided. Consequently, they require a complicated construction and increased cost. Further, it is troublesome to lead multiple wires from multiple bobbins to the upper wire positioning guides along multiple wire transport paths, including stretching the wires over a number of rollers by hand in order to prepare for machining.
Japanese Laid-Open Patent Publication No. 2-279221 discloses a wire-cut electroerosion apparatus which is provided with a wire joining means which precedes a wire tensioning device. A clamping means fixed to a carriage clamps the top of a new wire which has been pulled out from a spare wire bobbin. A first wire electrode is stopped and then clamped by the clamping means. The carriage then moves to cross the first wire and the new wire. The wire joining means joins the first wire to the new wire. Then a cutter which precedes the wire joining means in the direction of wire travel, cute the joined wire off of the first wire bobbin. The joint of the two wires is fed forward along the wire transport path, which includes the wire tensioner. A pair of joint cutters cut the joint before it reaches a pipe guide which is provided for guiding the wire into an upper wire positioning guide and through the work zone. After the joint is removed, the wire is supplied into the pipe guide.
Thus, in the disclosed device, two wires are crossed and joined together. The device has an advantage in that the wire joining operation is easy. However, the cross sectional area of the joint is larger than a single wire diameter, and is similar to a joint created by a thread splicing device in a spinning machine. Moreover, two surplus wire pieces extending beyond the joint are left over. Therefore, the joint between the wires may be snatched or snagged by rollers or by a wire tensioner positioned along the wire transport path. Further, the joint must be removed in order for the new wire to pass through the pipe guide, and/or wire guide, and into the machining slit in the workpiece. In an automatic wire threading device using a pipe guide, the inner diameter of the pipe guide is only slightly larger than the wire diameter in order to precisely guide the leading end of wire into a start hole or the machining slit in the workpiece. For example, the inner diameter of the pipe guide is generally about 1 mm when using a 0.2 mm diameter wire. In general, the disclosed device only provides a wire-cut electroerosion apparatus having an automatic wire threading device for making it possible to-cut multiple closed contours on the workpiece or as a precaution against wire breakage.
However, the wire joining means of the above disclosure has no provision for detecting whether the two wires have been reliably joined together. Moreover, no means for automatically resetting the wires to retry the wire joining operation, for example, if the wires burn out at the cross or winding point due to a large current flow or separate because the wires are not sufficiently joined together, is provided.
In addition, this apparatus includes a structure in which a new wire can be pulled out only from a right or a left bobbin. Therefore, it is impossible to perform a next wire change, even if the empty bobbin is replaced with a new spare wire bobbin. Thus, automatic and continuous machining operations, which require continuously supplying a spare wire bobbin, are difficult.
Further, the above disclosed wire joining means did not always provide sufficient operational reliability when used as a wire refeeding device, making it difficult to realize complete automatic operation.